Examples of a method for producing a useful protein in a plant includes a method of using a transformed plant in which a foreign gene is introduced into a cell, and a method of infecting a plant cell with a virus vector. The method using a virus vector is advantageous since it provides higher expression efficiency than the method using a transformed plant.
Non Patent Document 1 discloses a method for expressing a foreign gene in a plant cell by infecting the plant cell with at least two agrobacteria into which virus vectors are introduced respectively. This method eliminates the need for creating a construct for each of plural genes when combinations of various genes are tested to find a combination for encoding a useful protein. Therefore, this method is useful in analyzing functions or the like of a large number of proteins. Further, a virus vector disclosed in Non Patent Literature 1 can realize high expression speed, can be constructed at a low cost, and can eliminate steps of a conventional gene recombination process.
It is desired that a useful protein produced in a plant can be produced efficiently and in mass scale since it is used not only for food, but also for medical products. In view of this, the inventors of the present invention have constructed a system for producing a protein by using a virus vector (see Patent Literatures 1 through 3).
As another system for producing a protein by using a virus vector, Patent Literature 4 discloses a method for increasing a production amount of a useful protein by improving efficiency of producing a transcription product. Patent Literature 4 discloses a method for expressing a target protein by inserting an intron sequence into a replication sequence of a virus vector, and introducing the virus vector thus obtained into a host cell. According to this method, in which an intron region containing a lot of adenosine (A), and thymidine (T) or uracil (U) is removed from the replication sequence of the virus vector or is substituted with an intron derived from a plant cell so that (i) decomposition of the transcription product in the plant cell can be suppressed and (ii) efficiency of producing the transcription product can be improved, it is possible to increase a production amount of a target protein.
Meanwhile, each of Non Patent Literatures 2 and 3 discloses a method for improving replication efficiency of a vector which is introduced in a host cell and which contains a base sequence of potyvirus. According to the method disclosed in Non Patent Literatures 2 and 3, an intron sequence is inserted into the base sequence of potyvirus so that a transformed sequence is obtained, and a vector containing the transformed sequence is introduced into Escherichia coli. In the E. coli to which the vector is introduced, introduction of intron suppresses expression of a virus protein encoded by the base sequence of potyvirus. This controls toxic influence of the virus on the E. coli, and attains better growth of the E. coli, thereby improving replication efficiency of the vector in the E. coli. 